1. Field
This present invention relates to a synthesis method for producing iron-containing active materials capable of retaining its surface area at high temperature and to be used in the metal electrode of rechargeable oxide-ion battery (ROB) cells.
2. Description of Related Art
Electrical energy storage is crucial for the effective proliferation of an electrical economy and for the implementation of many renewable energy technologies. During the past two decades, the demand for the storage of electrical energy has increased significantly in the areas of portable, transportation, load-leveling and central backup applications.
The present electrochemical energy storage systems are simply too costly to penetrate major new markets. Higher performance is required, and environmentally acceptable materials are preferred. Transformational changes in electrical energy storage science and technology are in great demand to allow higher and faster energy storage at lower costs and longer lifetimes necessary for major market enlargement. Most of these changes require new materials and/or innovative concepts, with demonstration of larger redox capacities that react more rapidly and reversibly with cations and/or anions.
Batteries are by far the most common form of storing electrical energy, ranging from: standard every day lead-acid cells; nickel-metal hydride (NiMH) batteries, taught by Kitayama in U.S. Pat. No. 6,399,247 B1; metal-air cells taught by Isenberg in U.S. Pat. No. 4,054,729, to the lithium-ion battery taught by Ohata in U.S. Pat. No. 7,396,612 B2. Most of these latter battery cells require liquid electrolyte systems.
Batteries range in size from button cells used in switches, to megawatt load leveling applications. They are, in general, efficient storage devices, with output energy typically exceeding 90% of input energy, except at the highest power densities.
Rechargeable batteries have evolved over the years from lead-acid through nickel-cadmium and nickel-metal hydride (NiMH) to lithium-ion batteries. NiMH batteries were the initial workhorse for electronic devices such as computers and cell phones, but they have almost been completely displaced from that market by lithium-ion batteries because of the latter's higher energy storage capacity. Today, NiMH technology is the principal battery used in hybrid electric vehicles, but it is likely to be displaced by the higher power energy and now lower cost lithium batteries, if the latter's safety and lifetime can be improved. Of the advanced batteries, lithium-ion is the dominant power source for most rechargeable electronic devices.
What is needed is a dramatically new electrical energy storage device that can easily discharge and charge a high capacity of energy quickly and reversibly, as needed. What is also needed is a device that can operate for years without major maintenance. What is also needed is a device that does not need to operate on natural gas, hydrocarbon fuel or its reformed by-products such as H2. One possibility is a rechargeable oxide-ion battery (ROB), as set out, for example, in U.S. application Ser. No. 13/167,900, filed Jun. 24, 2011, and U.S. Patent Publication No. 2011/0033769 A1 (Huang et al.).
A ROB essentially is an oxygen-concentration cell, and it comprises a metal electrode, an oxide-ion conductive electrolyte, and an air cathode. The metal electrode undergoes reduction-oxidation cycles during charge and discharge processes for energy storage. The working principles of a rechargeable oxide-ion battery cell 10 are schematically shown in FIG. 1. In discharge mode, oxygen molecules are electrochemically reduced into oxide ions on air electrode 12 by the cathodic reaction of x/2O2+2xe−→xO2−. The oxide ions migrate from the air electrode (high oxygen partial pressure side) to the metal electrode (14, low oxygen partial pressure side) through the electrolyte 16 under the driving force of gradient oxygen chemical potential. In principle, there exist two possible reaction mechanisms to oxidize the metal. One of them, solid-state diffusion reaction designated as Path 1, is that oxide ion can directly electrochemically oxidize metal to form metal oxide. The other, gas-phase transport reaction designated as Path 2, involves generation and consumption of gaseous phase oxygen specie. The reactive interface 18, that converts oxide ions into gaseous phase oxygen species, locates in the vicinity of metal electrode-electrolyte interface. The oxide ion can be initially converted to a gaseous oxygen molecule on a metal electrode, and then further reacts with metal via solid-gas phase mechanism to form metal oxide. In charge mode, the oxygen species, released by reducing metal oxide to metal via electrochemical Path 1 or solid-gas mechanism Path 2, are transported from the metal electrode back to the air electrode.
As one of the key components in a ROB metal electrode, the metal (Me) plays a reservoir role in uptaking or releasing oxygen during discharge-charge cycle via the electrodic reaction of Me+xO2−MeOx. The Me in a ROB is selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Y, and W, preferably Mn, Fe, Mo, and W, more preferably Fe. The metal oxidation kinetics, if controlled by bulk diffusion of active species through dense oxide scale, can be depicted by parabolic law of (λw)2=kgt (eq. 1) where λ is weight fraction of oxygen in oxide, W the weight gain per surface area in g/cm2, kg the parabolic reaction constant in g2/cm4/s, and t the reaction time in s (second). Provided that the surface area of active material is A (cm2/g) and total weight gain of W in gram, then
                    w        =                              W            A                    .                                    (                  eq          .                                          ⁢          2                )            Combining the above two equations and make derivative W with respect to t, then one yields
                                          ⅆ            W                                ⅆ            t                          =                  A          ⁢                                                    k                g                                                    2              ⁢              λ                                ⁢                                    t                              -                                  1                  2                                                      .                                              (                  eq          .                                          ⁢          3                )            
      ⅆ    W        ⅆ    t  can then further be mathematically derived into maximum electrical current I (ampere) using the formula of
                    I        =                                            NF              Z                        ⁢                                          ⅆ                W                                            ⅆ                t                                              =                      A            ⁢                                                  ⁢                          nF              Z                        ⁢                                                            k                  g                                                            2                ⁢                λ                                      ⁢                          t                              -                                  1                  2                                                                                        (                  eq          .                                          ⁢          4                )            where n is the number of electrical charges involved in the oxidation reaction, F the Faraday constant of 96485 coulumb/mol, and Z the formula weight of the oxide. Equation 4 clearly suggests that the maximum electrical current I is proportional to A. The larger A, the higher I. Clearly increasing surface area (A) of the active materials emerges as one of leading solutions to enhance overall metal redox reactions and consequently boost cell performance in terms of current during charge-discharge operation for energy storage. Thus, fine iron particles are preferred for ROB application. Unfortunately, directly handling and processing fine metal particles including Fe imposes serious risk due to increasing fire danger with the decreasing size of metal particles. In addition, even if safety measures prudently implemented enable utilization of finer metal powder for example Fe, the loss of surface area of Fe particles at high temperature may lead to the degradation of cell performance of a ROB over an extended period time of operation. The loss of surface area is the consequence of densification and/or coarsening of the materials driven by minimization of its surface energy.
Therefore, there is an urgent need to develop synthesis methods enabling the formation of active materials containing fine Fe particles so that the Fe particles can be handled at a relatively safe manner despite its fairly microscopic size during cell assembly. Also, the fine Fe particles in the materials possess significant resistance against coarsening and densification to preserve its surface area over time at high temperature (600° C.-800° C.).
It is one of the main objects of this invention to provide a solid-solution method for providing Fe-containing active material for use, generally, in a metal electrode of rechargeable oxide-ion battery (ROB) cells.